Exception Handling in Python

Syntax errors and exceptions, the built-in exceptions, raise and assert, and the order in which try, except, else and finally run.

Key ideas

A syntax error (or an IndentationError) is reported before the program runs; an exception is raised while a correct program runs.
When a line in try raises an exception, the rest of try is skipped and the first matching except runs.
else runs only when try finished with no exception; finally always runs, even just before a function returns.
After the handler and finally, the program carries on with the line after the whole try statement.
An exception that no except matches stops the program, once finally has run; an except with no name catches every exception.
raise throws an exception on purpose; assert condition raises an AssertionError when the condition is False.

Remember

ZeroDivisionError	dividing by zero with /, // or %
ValueError	right type, wrong value: int('ten'); L.remove(x) when x is not in L
TypeError	the wrong type: '5' + 3; len(42); changing a tuple
IndexError, KeyError, NameError	an index out of range; a missing dictionary key; a name never given a value
EOFError, IOError, ImportError	reading past the end (pickle.load); a file problem; a missing module
The order	no exception: try, else, finally; an exception: try up to it, except, finally

Watch out

Thinking else runs after an exception has been handled: it runs only when there was none.
Forgetting that finally runs in every case.
Calling a missing colon a runtime error: it is a SyntaxError, found before the program runs.
Expecting the rest of try to run after the line that raised the exception.

Worked example

Write the letters this prints, in order, separated by commas.
try:
    x = 2
    y = x / 0
    print('A')
except ZeroDivisionError:
    print('B')
else:
    print('E')
finally:
    print('C')
print('D')

Working: x / 0 raises ZeroDivisionError, so A and else are skipped; finally always runs; then the program carries on

Answer: B, C, D